Modulators using solid state switches, magnetic switches, and combinations of the two have been used in recent years for driving magnetron transmitters in radar systems. These devices have been capable of generating pulses no shorter than 100 nanoseconds in duration.
In prior art devices a saturable reactor or transformer is used to perform the final, fast switching which generates the output pulse. The present invention uses a unique, lumped constant delay line composed of series inductances and shunt capacitors. Each series inductance is a saturable reactor switch. Two to five sections have been used, with each section consisting of a series saturable reactor and a shunt capacitance. The initial switching is performed by the string of series SCR modules, across which is connected the lumped constant delay line. The switching transient travels down the delay line as the saturable reactors switch in sequence. With each switching action the transient is sharpened and increased in amplitude. The load, in this case the primary of the pulse transformer, is connected in series with the last section of the delay line. When the last saturable reactor switches, a very short pulse is applied to the magnetron.
In prior art devices, the low impedance level of the switching modules is matched to the high impedance of the magnetron by one or more pulse transformers having a high, overall step-up ratio. This adds series inductance and shunt capacitance to the discharge circuit and sets a lower limit on pulse duration. The present invention uses a number of switching modules in series. The voltage and impedance level of the series combination is proportional to the number of modules used. The higher impedance of the switching circuit can now be matched to the magnetron using a pulse transformer having a relatively low step-up ratio. This reduces series inductance and shunt capacitance and makes it possible to generate shorter pulses.
Some solid state, magnetic modulator circuits typically require a bias current in the saturable transformer which couples the low level pulse generator to the output pulse generator circuit. The required value must be controlled in a closed loop servo system. The present invention has no bias requirements.